The present studies are undertaken to expand our knowledge of the relationship between nutrition and plasma lipoprotein levels, particularly levels of high density lipoprotein (HDL). Because HDL levels are negatively correlated with risk of coronary heart disease, there is a great deal of interest in understanding how HDL levels vary according to nutritional pattern. Because the mechanisms linking HDL levels to atherogenesis are poorly understood, it is equally important to understand just how nutritional components affect HDL levels. Moreover, risk is related not only to HDL but also to very low density lipoprotein (VLDL) and low density lipoprotein levels (LDL). The studies proposed, therefore, involve measurement not only of multiple components of HDL but also of VLDL and LDL and exploration of the mechanisms underlying changes in HDL levels, including measurements of the turnover of HDL apoproteins. The nutritional variables to be studied include: diurnal variations in HDL; minimal cholesterol absorption (zero cholesterol intake plus administration of plant sterol esters); high cholesterol intake (1000 mg/d); changes in absolute level of fat intake; changes in the ratio of polyunsaturated to saturated fatty acids; the effects of obesity and of marked shifts in calorie balance; the effects of dietary intake of phospholipids. Finally, a novel approach to studying the possible contribution of chylomicrons to HDL in man is proposed involving sustained intraduodenal infusions of triglycerides and/or phospholipids to study consequences with respect to HDL concentration and composition.